Advanced radiotherapy requires precise imaging for treatment planning. Radiotherapy encompasses therapy, treatment and imaging (and the like), and imaging includes CT and ultrasonic imaging. Since the imaging and the treatment take place at different times and/or locations, patient positions must be maintained as identical as possible in the two procedures to reduce positional errors associated with internal organ displacement and deformation. Patient rotation about a vertical rotational axis is acceptable since such a rotation has no effect on the gravitational forces applied to the internal organs and thus no effect on organ displacement and deformation. Similarly, the patient may be translated relative to a radiation beam but remain in the same position relative to the gravitational field.
However, irradiating soft, unsupported tissues in the upright mode is not simple, due to, among other things, the problem of immobilization. For example, prostate irradiation in the upright mode incorporates positioning the prostate so as to intersect a generally horizontal and stationary radiation beam, and rotating an upright patient about a vertical rotational axis. Precise prostate positioning and immobilization relative to the beam is required. Immobilization must be both internal (the prostate relative to the pelvic boney structures) and external (the boney structures relative to the radiation beam). External immobilization of an upright patient for the duration of a treatment is a challenge as long as the patient is on his feet.